Using scissors, cut along back shell of each prawn all the way up to tail, exposing vein; pull out vein. Turn prawns over. Using small sharp knife and starting just below tail end, butterfly each prawn by cutting 1/4-inch-deep slit to opposite end. Place prawns, shell side down, on rimmed baking sheet. (Can be made 6 hours ahead. Cover; chill.)

Meanwhile, spread 1 teaspoon morel butter over each giant prawn (or 1/4 teaspoon over each of 24 large prawns). Roast just until prawns are opaque in center, about 7 minutes for giant prawns or 4 minutes for large prawns. Top prawns with remaining morel butter, dividing equally. Return to oven just long enough to melt butter slightly, about 10 seconds.